In recent years, hot dip galvannealed steel sheet has been used in large quantities for automobiles etc. This hot dip galvannealed steel sheet is usually produced by the Sendzimir method or the non-oxidizing furnace method, but after cold rolling has to be heated to an 800° C. or so high temperature and cannot be overaged like with a continuous annealing line after plating. For that reason, in the case of soft low carbon Al-killed steel or B-containing low carbon Al-killed steel, solute C remains in a large amount. Compared with cold rolled steel sheet produced by the cold rolling-continuous annealing process, the yield strength is high, yield point elongation easily occurs, the elongation is low, and workability is otherwise degraded unavoidably. Specifically, in terms of elongation, 4% or more deterioration occurs.
On the other hand, Japanese Patent No. 2783452 discloses a method of production of hot dip galvannealed steel sheet preplating the sheet with Ni, then rapidly heating it to 430 to 500° C., galvanizing it, then alloying it. In the case of this method, even at a high temperature, it is only necessary to raise the temperature to the 550° C. or so at the time of alloying. As the raw sheet, it is possible to use cold rolled steel sheet produced by the cold rolling-continuous annealing process. However, in cold rolled steel sheet, to prevent the occurrence of stripe patterns called coil break and correct the shape, the usual practice is to perform temper rolling at a 0.6 to 1.5% or so elongation rate. When passing cold rolled steel sheet of low carbon Al-killed steel subjected to that extent of temper rolling through a galvanization process using the above Ni preplating method, the solute C adheres to the movable dislocations at the time of a temperature rise and the workability deteriorates in a “strain aging phenomenon”.